Method for enabling movement of a centralized pipe through a reduced diameter restriction and apparatus therefor

ABSTRACT

A centralizer for laterally positioning a pipe in an opening larger in diameter than an opening through which the centralizer may freely pass is disclosed. The centralizer includes a plurality of helically shaped spring blades affixed at each end thereof to a slip collar. The slip collars are adapted to slide and rotate about the exterior surface of the pipe. A retaining device is disposed axially between the slip collars and is adapted to limit axial motion of the slip collars. One embodiment of the retaining device is a sleeve which is adapted to be affixed to the exterior surface of the pipe at a selected axial position.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation in part of application serial No.09/824,928 filed on Apr. 3, 2001 and assigned to the assignee of thepresent invention.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The invention is related generally to the field of centralizers,such as used on casing inserted in wellbores drilled through the earth.More specifically, the invention is related to centralizers which canpass through an opening that is smaller than the opening in which adevice is to be centralized.

[0005] 2. Background Art

[0006] Wellbores drilled through the earth to extract petroleum and thelike are commonly “completed” by cementing a steel pipe or casing in thewellbore after it is drilled. The casing serves to maintain themechanical integrity of the wellbore, provides a conduit for producedfluids to move to the earth's surface, and hydraulically isolates earthformations from each other so that high fluid pressure earth formationsdo not discharge fluid into lower fluid pressure earth formations.

[0007] The casing is typically inserted into the drilled wellbore bycoupling segments of the casing together and lowering the coupledsegments into the wellbore. To cement the casing in place in thewellbore, cement is typically pumped through the interior of the casing,and is discharged into an annular space between the casing and thewellbore from the bottom of the casing. An important aspect of properlycementing the casing in place to complete a wellbore is that the casinghave a substantially uniform annular space around it at all places alongthe length of the wellbore. Uniformity of the annular space increasesthe likelihood that the cement will completely and uniformly fill theannular space, thereby ensuring that the wellbore properly hydraulicallyisolates earth formations from each other. Uniformity of the annularspace is affected by the trajectory of the wellbore and the final shapeof the wellbore, among other factors. Frequently wellbores are drilledalong trajectories other than vertical, so earth's gravity and bends inthe wellbore cause the casing to rest on the wall of the wellbore insome places along the wellbore. In other cases, the wall of the wellboremay include out of round sections, for example washouts or keyseats,which make cementing operations more difficult.

[0008] It is known in the art to use centralizers to keep the casing asclose as possible to the center of the wellbore for proper cementing.Typical centralizers known in the art are shown, for example, in a salesbrochure published by Antelope Oil Tool & Manufacturing Company, MineralWells, Tex. (not dated). Centralizers are typically coupled to theexterior surface of the casing at selected locations along the casingprior to inserting the casing into the wellbore. Spring blades on thecentralizers provide a restoring force which tends to push the casinginto the center of the wellbore. Specifications for the amount ofrestoring force, and proper use of centralizers are described in adocument entitled, Specifications for Bow-Spring Centralizers, APISpecification 10D, fifth edition, American Petroleum Institute, Wash.,D. C. (1994). Generally speaking, casing centralizers are made to centera particular outside diameter (OD) casing within a particular nominaldiameter wellbore. The casing OD is selected by the wellbore operator toclosely match the wellbore diameter, which is primarily related to thediameter of the drill bit used to drill that segment of the wellbore.

[0009] More recently, it has become known in the art to drill wellboresto a depth greater than a depth to which casing has been set, in whichthe greater depth portion of the wellbore has a diameter larger than thediameter of the casing. This type of drilling can be performed usingvarious types of reaming tools such as hydraulic underreamers orspecialized drill bits known as bi-center bits. See, for example, U.S.Pat. No. 6,039,131 issued to Beaton. Drilling this type of wellboremakes it possible to insert a larger completion device in the deeperportion of the wellbore, such as gravel pack or sand screens, than wouldbe possible using conventional drilling techniques. Completing wellboreshaving such deeper sections including oversize diameters usingcentralizers known in the art has proven difficult because it isimpracticable to move a larger outside diameter centralizer through asmaller internal diameter casing or other opening.

[0010] It is desirable to have a centralizer which can position a casinginside a larger diameter wellbore than the opening through which thecentralizer can freely pass. One such type of centralizer known in theart includes a plurality of bow shaped spring blades coupled at each endthereof to a collar. One of the collars is affixed to an exteriorsurface of the casing. The other collar is allowed to “float” or moveaxially along the outer surface of the casing. Typically, the exteriorsurface of the casing will be machined or otherwise reduced in diameterunder the position of the spring blades. The centralizer is thus able tolaterally compress when the centralizer is pushed through an openingsmaller than the uncompressed diameter of the spring blades. When thespring blades are compressed, they extend a longer distance along thelength of the casing. The floating collar is able to move in responsethereto. When the compressible centralizer is moved into a largerdiameter opening after passing through the restriction, the springblades expand laterally, providing a restoring force to centralize thecasing inside the larger diameter opening.

[0011] A limitation to the laterally compressible centralizer known inthe art is that it requires a relatively high starting force and runningforce to move axially through the restricted diameter opening.

SUMMARY OF THE INVENTION

[0012] One aspect of the invention is a centralizer for laterallypositioning a pipe in an opening larger in diameter than an openingthrough which the centralizer may freely pass. The centralizer includesa plurality of helically shaped spring blades affixed at each endthereof to a slip collar. The slip collars are adapted to slide androtate about the exterior surface of the pipe. A retaining device isdisposed axially between the slip collars, and is adapted to limit axialmotion of the slip collars. In one embodiment, the retaining device is asleeve which is adapted to be affixed to the exterior surface of thepipe at a selected axial position.

[0013] In one embodiment, the slip collars include grooves in theexterior surface adapted to receive the ends of the spring blades. Inone embodiment, the sides of the grooves are radiused to avoiddistorting the slip collars. In one embodiment, a thickness of the slipcollar material below the groove is substantially the same as that ofthe retaining sleeve. In one embodiment, the centralizer includes ananti-friction bearing between at least one of the slip collars and theretaining device. The anti-friction bearing is adapted to reducerotational friction of the slip collar about the casing when the springblades are laterally compressed. In one embodiment of a sleeve typeretaining device, the, a sleeve includes a plurality of set screwsarranged in a helical pattern corresponding to the shape of the springblades to enable access to the set screws. In one embodiment, theretaining device is a sleeve assembled from circumferential segments. Inthis embodiment, the segments are affixed to the exterior surface of thepipe using an adhesive such as epoxy, or by welding. In one embodiment,the slip collars include mud channels on an exterior surface to avoidsurge and swab effects when running the centralizer.

[0014] Other aspects and advantages of the invention will be apparentfrom the following description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 shows a laterally compressible centralizer assemblyaccording to one aspect of the invention.

[0016]FIG. 2 shows a cross section of one end of an embodiment of thecentralizer.

[0017]FIG. 3 shows a partial side view of a particular embodiment of oneof the slip collars.

[0018]FIG. 4 shows an end view of the slip collar embodiment shown inFIG. 3.

[0019]FIG. 5 shows two centralizers according to the invention affixedto a pipe being passed through a restricted diameter opening.

[0020]FIG. 6 shows a particular embodiment adapted to pass through anopening substantially smaller than the opening in which the pipe is tobe laterally positioned.

[0021]FIG. 7 shows a particular embodiment of the retaining sleeve.

[0022]FIG. 8 shows the embodiment of FIG. 6 partially disposed insideone end of an outer casing string.

DETAILED DESCRIPTION

[0023]FIG. 1 shows a laterally compressible centralizer according to oneaspect of the invention. The centralizer is placed on the exterior of ajoint of casing 10 which is to be moved through an opening having asmaller diameter than the opening in which the centralizer ultimatelywill be positioned. Such smaller diameter openings may include, forexample, a casing string in a wellbore drilled through the earth, belowwhich open wellbore is “underreamed” to a larger diameter than theinternal diameter of the casing string. The centralizer includes slipcollars 14 disposed on the exterior of the casing 10 at axially spacedapart locations thereon. The slip collars 14 are interconnected by “bow”shaped spring blades 16, of a type conventional for bow spring typecentralizers. Each of the spring blades 16 is affixed at each of itsends to one of the slip collars 14. In their uncompressed state, thespring blades 16 extend laterally a predetermined radial distance beyondthe radius of the casing 10. This distance, and the number of springblades 16 are selected based on factors such as the diameter of theopening into which the casing 10 is to be centralized and the weight ofthe casing 10, among other factors. In this embodiment, the springblades 16 are preferably helically shaped. More preferably, the helicalangle of each spring blade 16 is within a range of about 15 to 20degrees away from being parallel to the longitudinal axis (not shown) ofthe casing 10. The spring blades 16 may be welded, adhesively bonded, orotherwise affixed to the slip collars 14. The spring blades 16 and slipcollars 14 may also be formed from a single sheet, tube, or other pieceof material. The material from which the spring blades 16 are formed istypically steel, but may be any other suitable material capable ofproviding the required restoring force.

[0024] The centralizer in this embodiment includes a retaining deviceadapted to be affixed to the exterior surface of the casing 10 at aselected axial position along the exterior surface. In this embodiment,the retaining device is a sleeve 12 which is affixed to the exteriorsurface of the casing 10. The retaining sleeve 12 in this embodimentincludes a plurality of set screws 18 which are disposed inside threadedholes to enable tightening against the exterior of the casing 10. Inthis embodiment, the set screws 18 are disposed in a helical patternabout the exterior of the sleeve 12. The helical pattern of the setscrews 18 is preferably selected so that they are easily accessiblebetween the individual spring blades 16. Although set screws are shownin this embodiment to retain the sleeve 12 on the exterior of the casing10, other means for retaining the retaining sleeve may be used,including welding and/or adhesive. Other embodiments of the retainingsleeve 12 will be further explained. The retaining sleeve shown in FIG.1 is only one embodiment of a retaining device. Other embodiments ofretaining device may be used in accordance with the invention. Generallyspeaking, the retaining device is intended to limit axial motion of theslip collars 14, while presenting an external diameter sufficientlysmall to enable lateral compression of the spring blades 16 when thecentralizer is moved through a small diameter opening. Other embodimentsof the retaining device will be further explained.

[0025] An outer edge of at least the lowermost slip collar 14 preferablyincludes thereon a beveled or tapered edge 14A to reduce the possibilityof the centralizer becoming caught on any protrusions or shoulders in awellbore (not shown) during movement of the casing 10 therethrough.

[0026] The edges of the retaining sleeve 12 preferably include thereonan anti-friction bearing 20 which may be a roller or ball bearing, a lowfriction coating or any other device which is adapted to reduce frictionof rotation between the slip collar 14 and the retaining sleeve 12 whenthe edges of the slip collar 14 and retaining sleeve 12 come intocontact. The reason for this contact will be further explained.Alternatively, the anti-friction bearing 20 may be formed or attached tothe corresponding edge of the slip collars 14. Additionally, ananti-friction bearing surface 22 may be applied to or otherwise formedinto the exterior surface of the casing 10, or on a correspondinginterior surface of the slip collars 14, to enable the slip collars 14to slide axially along the casing 10 and to rotate about the exterior ofthe casing 10.

[0027] A cross-sectional view of one end of the centralizer as affixedto the exterior of the casing 10 is shown in FIG. 2. In the embodimentshown in FIG. 2, the slip collars 14 preferably include therein a slotor groove 15 adapted to receive the end of each one of the spring blades16. As shown in FIG. 2, preferably, the groove 15 is formed in the outersurface of the slip collar 14, such as by milling, grinding, cutting(such as by laser) or preforming into the exterior surface of the slipcollar material, so that the spring blade 16 is substantially flush withthe exterior surface of the remainder of the slip collar 14 wheninserted into the groove 15. The groove 15 preferably also has a depthselected so that the thickness of a tongue portion 17 thereinsubstantially matches the thickness of the retaining sleeve 12. Thisconfiguration enables the spring blades 16 to compress laterally tonearly as small a diameter as the external diameter of the slip collars14, without binding or sticking on the outer surface of the sleeve 12.Because in this embodiment, the spring blades 16 are flush with theexterior of the slip collars 14, the diameter of the restricted openingthrough which the centralizer may pass in its compressed state isminimized. This feature increases the useful diameter range of thecentralizer.

[0028]FIG. 3 shows a side view of a segment of one of the slip collars14 to explain another aspect of a centralizer according to theinvention. The grooves 15 in this embodiment of the slip collar 14 havetherein radiused sides 19. The radiused sides 19 preferably havesubstantially the same width as the spring blades 16 at the ends of theblades, but increase in width towards the tongue end (17 in FIG. 2). Theradiused sides 19 better enable twisting of the spring blades 16, as aresult of relative rotation of the slip collars 14, without distortingthe shape of the slip collars 14. The term “radiused” as used herein todescribe the shape of the sides of the grooves 15 is meant to includewithin its scope any form of curved or tapered change in width along thelength of the groove such that the spring blades 16 may twist around theaxis (not shown) of the casing (10 in FIG. 1) substantially withoutdistorting the shape of the slip collars 14. Accordingly, the exactshape of the radius in the grooves shown in FIG. 3 is not intended tolimit the scope of the invention.

[0029] The embodiment shown in FIG. 3 also includes “mud channels” shownat 21, which may be slots or grooves, cut or otherwise formed into theexterior surface of the slip collar 14. The mud channels 21 arepreferably located on the circumference of the slip collars 14 betweenthe spring blade grooves 15, and are cut to a depth such that thethickness of the remaining portion of the slip collars 14 underneath themud channels 21 is substantially the same as that of the retainingsleeve (12 in FIG. 1). The mud channels 21 enable passage of fluid bythe slip collars 14 when the centralizer is inserted into a restrictedopening having an internal diameter only slightly larger than theexternal diameter of the slip collars 14. The mud channels thereforereduce “surge” and “swab” effects as the centralizer is moved along theinside of the restricted opening (not shown). Surge and swab are termsused in the art to describe a piston-like hydraulic action when a memberis moved inside a wellbore. An end view of the embodiment of the slipcollar 14 is shown in FIG. 4.

[0030] Another embodiment of the retaining sleeve is shown in moredetail in FIG. 7. In this embodiment, the retaining sleeve 12 is formedfrom a plurality, in this case four, interlocking segments 12A. Thesegments 12A may be affixed to the exterior surface of the casing (10 inFIG. 1) using epoxy or similar adhesive, or may be welded or otherwiseaffixed to the exterior surface of the casing (10 in FIG. 1). Theembodiment of the segments 12A shown in FIG. 7 includes interlockingstructures on circumferential ends of each of the segments 12A. Suchstructures may be in the form of a tongue 12C at one circumferential endof the segment 12A which fits into a corresponding slot 12B in a matingcircumferential end of another segment 12A. Tongue and slot interlockingstructures are only one example of interlocking structures which may beused in other embodiments of the retaining sleeve. Generally speaking,the interlocking structures are intended to maintain proper axial andcircumferential alignment of the segments 12A as they are affixed to theexterior surface of the casing (10 in FIG. 1). By maintaining properaxial and circumferential alignment, the bonding of the segments 12A tothe casing can be improved, particularly when the segments are bonded tothe casing using adhesive such as epoxy.

[0031] Having explained the general structure of a centralizer accordingto the various aspects of this invention, the manner in which theinvention is used in a wellbore will now be explained. Referring to FIG.5, when the sleeve 12 and centralizer (including slip collars 14 andspring blades 16) are attached to a joint of casing 10, the springblades extend to an uncompressed diameter D1. When the joint of casing10 is to be passed through a restricted diameter opening (such as acasing string 10A in a wellbore), typically the joint of casing 10 willbe coupled to similar joints of casing, some of which may includecentralizers such as shown in FIG. 1 and previously explained. As thejoint of casing 10 is moved into the restricted diameter opening 10A,the spring blades 16 are laterally compressed, to D2 in FIG. 5, which isthe internal diameter of the restricted diameter opening 10A. Lateralcompression extends the blades 16 axially along the joint of casing 10.Because the slip collars 14 are free to slide along the casing 10, axialextension of the spring blades 16 is substantially unhindered. Further,the bearing surface (22 in FIG. 2) used in some embodiments reducesaxial friction between the slip collars 14 and the joint of casing 10.Note that the retaining sleeve 12 limits the axial movement of one ofthe slip collars 14, but does not limit the axial separation between theslip collars 14. Therefore, the axial position of the centralizer isultimately limited, but the spring blades 16 are substantially free tocompress because the axial separation of the slip collars is not limitedby the retaining sleeve 12. Because the spring blades 16 of theembodiment shown in FIG. 5 are also helically shaped, lateralcompression of the spring blades 16 also results in some rotation of theslip collars 14 about the exterior of the casing 10, as the springblades 16 twist under the lateral compression. The slip collars 14, aspreviously explained, are also free to rotate about the casing joint 10.Further, as one of the slip collars 14 comes into contact with the edgeof the sleeve 12, the anti-friction bearing used in some embodiments (20in FIG. 2) reduces rotational friction between the sleeve 12 and theslip collar 14. The combination of axial extension and twisting of thespring blades 16 enables the spring blades 16 to more easily compress tofit within the restricted diameter opening (not shown), and then tolaterally expand as the centralizer is moved into a larger diameteropening (not shown).

[0032] In the invention, it has been determined that using helicalspring blades 16 on a centralizer such as shown in FIG. 1 reduces theamount of axial force needed to move the centralizer and casing (10 inFIG. 1) through any particular restricted diameter opening, whileretaining enough lateral expansion force to provide enough restoringforce to lift the casing 10 away from the wall of the larger diameteropening. Such forces are referred to in the art as “starting force” forinitiating movement into the restricted diameter opening, and “runningforce” to continue movement along the restricted diameter opening.

[0033] As a practical matter, a centralizer as shown in FIG. 1 includingthe sleeve 12 therein may be made, for example, by welding or otherwiseaffixing the spring blades 16 to the slip collars 14 while the sleeve 12is disposed axially between the slip collars 14. This may be performed,for example, on a suitable diameter mandrel (not shown) for convenienceof positioning the sleeve 12 and slip collars 14 prior to welding thespring blades 16 thereon. The mandrel (not shown) may also be used tostore and transport the centralizer including the sleeve 12 thereinuntil it is to be affixed to the casing 10.

[0034] It is known in the art to affix a helical bow spring bladecentralizer so as to straddle a casing collar (not shown in the Figures)wherein the slip collars are free both to rotate and slide along thecasing. Typically, the centralizer used to straddle a casing collar isin the form of “half shells” or a “clam shell” which is opened to clamparound the casing. The centralizer and method for attaching to thecasing in the invention are different from the prior art “straddle” typecentralizers in that the straddle type centralizer is limited as to itsamount of lateral compressibility by the diameter of the casing collar.Typically, straddle type centralizers are not intended to be movedthrough a restricted diameter opening and therefore do not have theuseful diameter range of the centralizer of the present invention.Further, the position along the exterior of the casing is limited, whenusing straddle type centralizers, to the position of the casing collaritself. In embodiments of the invention, the retaining sleeve and thecentralizer can be positioned at any selected location along a “joint”of casing.

[0035] An embodiment of the invention which is adapted to pass throughan opening substantially smaller than the diameter of the opening inwhich a pipe is to be laterally positioned is shown in FIG. 6. The pipeto be laterally positioned may be a casing, for example, just as in theprevious embodiments. In FIG. 6, a segment of this casing on which acentralizer according to one embodiment of the invention is mounted isshown at 10B. In this embodiment, the segment of casing 10B may be afull length joint (about 30 feet or 10 meters in length) or may be aspecial, short-length segment (“pup joint”). In order to enable thecentralizer to be able to pass through an opening having a substantiallysmaller diameter than the opening in which the pipe 10B is ultimately tobe positioned, the present embodiment includes a reduced diametersection 10C formed on the exterior surface of the pipe segment 10B. Thereduced diameter section 10C may be formed by any method known in theart, including milling or grinding. Having the reduced diameter section10C enables the centralizer to be formed using blades 18 havingsufficient thickness to provide adequate strength and restoring force,while enabling, in some embodiments, compression of the centralizer to adiameter of not more than the nominal outer diameter of the pipe orcasing 10B. An example of the embodiment shown in FIG. 6 includes casinghaving a nominal outer diameter of 11.875 inches, wherein the casinghaving centralizers according to the invention pass through a pipestring having a drift (minimum) internal diameter of 12.25 inches. Thecasing is to be laterally positioned (centralized) within a hole, belowthe pipe string, having a nominal diameter of about 14.75 inches. Inthis example, it is contemplated that the reduced diameter section 10Chas a nominal outside diameter of about 11.5 inches. The foregoingdiameters and pipe sizes are meant only to be illustrative and are notin any way intended to limit the scope of the invention.

[0036] As in the other embodiments of the invention, the embodimentshown in FIG. 6 includes slip collars 14 adapted to slidingly, rotatablyfit on the exterior of the pipe 10B, a retaining device, which may be aretaining sleeve 12 mounted between the slip collars 14 on the exteriorof the pipe 10B, and helical bow springs 16 interconnecting the slipcollars 14. In some adaptations of the embodiment of FIG. 6, the slipcollars 14 may be adapted to fit on the reduced diameter section 10C. Inother adaptations, the slip collars 14 may be adapted to fit on thefull-diameter part of the pipe section 10B.

[0037] Another embodiment which can be inferred by viewing FIG. 6includes the retaining device 12 being integrally formed with the casing10. The retaining device 12 may be formed simply by leaving metal on theexterior of the casing 10 substantially undisturbed, while forming, suchas by grinding or milling, the reduced diameter section 10C on eitherside of the preferred position of the retaining sleeve. In suchembodiments using an integral retaining device, the slip collars 14 andaffixed bow springs are preferably assembled to the reduced diametersection 10C by coupling the slip collars 14 together fromcircumferential segments (not shown). Assembly of the slip collars maybe performed by welding the slip collar segments (not shown) together.If the slip collars are assembled from segments, preferably the segmentsinclude interlocking structures on the circumferential ends thereof toimprove axial and circumferential alignment during assembly. As in otherembodiments of the invention, the integrally formed retaining device maybe positioned at a selected axial location along the exterior surface ofthe casing.

[0038] The embodiment of FIG. 6 is shown in FIG. 8 as being moved intoan opening having a smaller internal diameter than the opening(borehole) in which the casing 10 is ultimately expected to becentralized or laterally positioned. The opening shown in FIG. 8 is oneend of an outer casing string 11. Typically, the internal diameter ofthe outer casing string 11 will be smaller than the hole in which thecasing 10 will be laterally positioned. As can be seen in FIG. 8, thespring blades 16 are laterally compressed by being moved into the outercasing string 11. Lateral compression of the spring blades 16 causes theslip collars 14 to move apart axially. Axial movement of the slipcollars 14 is limited by the retaining device 12. Advantageously, havingthe axial movement limited by the retaining device 12 enables relativelyfree and easy compression of the spring blades 16 irrespective of thedirection of motion of the casing 10 within the outer casing string 11.Some prior art compressible centralizers limited axial motion byaffixing one end of the spring blades to the outer surface of thecasing, or used the shoulder of a groove to limit axial motion. Suchdevices may have more difficulty in compressing the spring blades in oneor the other direction of motion.

[0039] While the invention has been described with respect to a limitednumber of embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the spirit of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

What is claimed is:
 1. A centralizer for laterally positioning a pipe inan opening larger in diameter than an opening through which thecentralizer may freely pass, comprising: a plurality of helically shapedspring blades affixed at each end thereof to a slip collar, the slipcollars adapted to slide and rotate about an exterior surface of thepipe; and a retaining device disposed axially between the slip collarsat a selected axial position along the exterior surface of the pipe soas to limit axial motion of the slip collars.
 2. The centralizer asdefined in claim 1 wherein an axial outermost edge of at least one ofthe slip collars comprises a taper adapted to reduce sticking of theslip collar as it is moved through the opening through which thecentralizer may freely pass.
 3. The centralizer as defined in claim 1wherein the retaining device comprises a sleeve including a plurality ofset screws therein to affix the sleeve to the pipe, the set screwsarranged in a pattern substantially conforming to a shape of the springblades whereby the set screws may be accessed through the blades.
 4. Thecentralizer as defined in claim 1 wherein the retaining device comprisesa sleeve adapted to be affixed to the exterior surface of the pipe. 5.The centralizer as defined in claim 4 wherein the sleeve is formed froma plurality of circumferential segments.
 6. The centralizer as definedin claim 5 wherein the segments comprise interlocking structures oncircumferential ends thereof.
 7. The centralizer as defined in claim 1wherein the spring blades define an angle of about 15 to 20 degrees withrespect to a longitudinal axis of the pipe.
 8. The centralizer asdefined in claim 1 further comprising an anti-friction bearing disposedbetween an edge of the retaining device and at least one of the slipcollars.
 9. The centralizer as defined in claim 1 further comprising ananti-friction bearing surface disposed between the retaining device andat least one of the slip collars.
 10. The centralizer as defined inclaim 1 wherein the spring blades are affixed to the slip collars incorresponding grooves on an exterior surface thereof, the correspondinggrooves having a depth selected to make an exterior surface of thespring blades substantially flush with an exterior surface of the slipcollars.
 11. The centralizer as defined in claim 10 wherein the depth ofthe corresponding grooves is selected so that a material thickness ofthe slip collar therein is substantially equal to a thickness of theretaining sleeve.
 12. The centralizer as defined in claim 10 whereinsides of the grooves are radiused.
 13. The centralizer as defined inclaim 1 wherein the slip collars comprise mud channels in an exteriorsurface thereof.
 14. The centralizer as defined in claim 1 wherein thespring blades and the slip collars are formed from a single piece ofmaterial.
 15. The centralizer as defined in claim 1 wherein theretaining device is adapted to fit on a pipe section having a smallerouter diameter than an outer diameter of the pipe to be laterallypositioned.
 16. The centralizer as defined in claim 15 wherein the slipcollars are adapted to fit on the pipe section.
 17. The centralizer asdefined in claim 16 wherein the slip collars and the spring blades areadapted to compress to at most the outer diameter of the pipe to belaterally positioned.
 18. The centralizer as defined in claim 1 furthercomprising a pipe section having an outer diameter smaller than an outerdiameter of the pipe to be laterally positioned, and wherein theretaining device is disposed on the pipe section.
 19. The centralizer asdefined in claim 18 wherein the slip collars are disposed on the pipesection.
 20. A method for laterally positioning a pipe in an openinglarger in diameter than an opening through which the pipe may freelypass, comprising: sliding a bow spring centralizer and a retainingdevice onto an exterior surface of the pipe, the centralizer comprisinga plurality of helically shaped spring blades affixed at each endthereof to a slip collar, the slip collars adapted to slide and rotateabout the exterior surface of the pipe, the retaining sleeve disposedaxially between the slip collars, the retaining sleeve adapted to beaffixed to the pipe so as to limit axial motion of the slip collars;affixing the retaining device to the exterior surface of the pipe; andinserting the pipe having the centralizer and retaining device attachedthereto into the opening through which the pipe may freely pass andthence into the opening having the larger diameter.
 21. The method asdefined in claim 20 wherein an axial outermost edge of at least one ofthe slip collars comprises a taper adapted to reduce sticking of theslip collar as it is moved through the opening through which thecentralizer may freely pass.
 22. The method as defined in claim 20wherein the retaining device comprises a sleeve including a plurality ofset screws therein to affix the sleeve to the pipe, the set screwsarranged in a pattern substantially conforming to a shape of the springblades whereby the set screws may be accessed through the blades. 23.The method as defined in claim 20 wherein the spring blades define anangle of about 15 to 20 degrees with respect to a longitudinal axis ofthe pipe.
 24. The method as defined in claim 20 further comprising ananti-friction bearing disposed between an edge of the retaining deviceand at least one of the slip collars.
 25. The method as defined in claim20 further comprising an anti-friction bearing surface disposed betweenthe retaining device and at least one of the slip collars.
 26. Themethod as defined in claim 20 wherein the spring blades are affixed tothe slip collars in corresponding grooves on an exterior surfacethereof, the corresponding grooves having a depth selected to make anexterior surface of the spring blades substantially flush with anexterior surface of the slip collars.
 27. The method as defined in claim26 wherein the depth of the corresponding grooves is selected so that amaterial thickness of the slip collar therein is substantially equal toa thickness of the retaining device.
 28. The method as defined in claim26 wherein sides of the grooves are radiused.
 29. The method as definedin claim 20 wherein the slip collars comprise mud channels in anexterior surface thereof.
 30. The method as defined in claim 20 whereinthe spring blades and slip collars are made from a single piece ofmaterial.
 31. The method as defined in claim 20 further comprising apipe section having an outer diameter smaller than an outer diameter ofthe pipe to be laterally positioned, and wherein the retaining device isdisposed on the pipe section.
 32. The method as defined in claim 31wherein the slip collars are disposed on the pipe section.
 33. Themethod as defined in claim 32 wherein the spring blades and slip collarsare adapted to compress to a diameter at most equal to the outerdiameter of the pipe to be laterally positioned.
 34. A centralizer forlaterally positioning a pipe in an opening larger in diameter than anopening through which the centralizer may freely pass, comprising: aplurality of helically shaped spring blades affixed at each end thereofto a slip collar, the slip collars adapted to slide and rotate about anexterior surface of the pipe; and a retaining device disposed axiallybetween the slip collars, the retaining device formed integrally withthe exterior surface of the pipe so as to limit axial motion of the slipcollars.
 35. The centralizer as defined in claim 34 wherein the pipecomprises reduced diameter segments on an exterior surface thereof, thereduced diameter segments adapted to receive the slip collars about anexterior surface thereof, the retaining sleeve formed by retaining atleast part of an exterior diameter of the pipe in a location axiallydisposed between the reduced diameter segments.
 36. The centralizer asdefined in claim 34 wherein an axial outermost edge of at least one ofthe slip collars comprises a taper adapted to reduce sticking of theslip collar as it is moved through the opening through which thecentralizer may freely pass.
 37. The centralizer as defined in claim 34wherein the spring blades define an angle of about 15 to 20 degrees withrespect to a longitudinal axis of the pipe.
 38. The centralizer asdefined in claim 34 farther comprising an anti-friction bearing disposedbetween an edge of the retaining device and at least one of the slipcollars.
 39. The centralizer as defined in claim 34 further comprisingan anti-friction bearing surface disposed between the retaining deviceand at least one of the slip collars.
 40. The centralizer as defined inclaim 34 wherein the spring blades are affixed to the slip collars incorresponding grooves on an exterior surface thereof, the correspondinggrooves having a depth selected to make an exterior surface of thespring blades substantially flush with an exterior surface of the slipcollars.
 41. The centralizer as defined in claim 40 wherein the depth ofthe corresponding grooves is selected so that a material thickness ofthe slip collar therein is substantially equal to a thickness of theretaining device.
 42. The centralizer as defined in claim 40 whereinsides of the grooves are radiused.
 43. The centralizer as defined inclaim 34 wherein the slip collars comprise mud channels in an exteriorsurface thereof.
 44. The centralizer as defined in claim 34 wherein thespring blades and the slip collars are formed from a single piece ofmaterial.
 45. The centralizer as defined in claim 34 wherein the slipcollars and the spring blades are adapted to compress to at most theouter diameter of the pipe to be laterally positioned.